During light phase of photosynthesis, ______ is oxidized and ______ is reduced.

In redox chemistry, oxidation is the loss of electrons and reduction is the gain of electrons. The light-dependent reactions of photosynthesis use light energy to pull electrons out of one substance and hand them to another, generating the ATP and NADPH that the plant later spends fixing carbon dioxide in the Calvin cycle.

1. Light absorbed at Photosystem II excites electrons in its reaction-centre chlorophyll to a higher energy level.

2. To replace the electrons Photosystem II passes onward, water molecules are split (photolysis): 2H2O → 4H+ + 4e− + O2. Water loses electrons in this step, so water is oxidized.

3. The released electrons travel down an electron-transport chain to Photosystem I, are re-energized by a second round of light absorption, and are finally delivered to NADP+.

4. NADP+ + 2e− + H+ → NADPH. NADP+ gains electrons in this step, so NADP+ is reduced to NADPH.

5. The first blank is therefore filled by water, the species oxidized, and the second blank by NADP, the species reduced.

As an independent check: the ATP and NADPH produced by these light reactions are spent to reduce CO2 into carbohydrate during the Calvin cycle, the light-independent reactions. CO2 is a reduction target of that later stage, never an oxidized species of the light phase itself, which rules out every pairing that places CO2 in the light phase.

Water is oxidized and NADP is reduced during the light phase, matching the pairing water, NADP.